(1) Field of the Invention
The present invention relates to a method of redrawing a metal cup. More particularly, the present invention relates to a redrawing method of preparing a uniformly thickness-reduced can barrel from a preliminarily drawn cup of a metal sheet, especially a coated metal sheet, while moderating the damage of the metal sheet and the coating layer and controlling the advance of the work hardening.
(2) Description of the Related Art
The production of a seamless can barrel by subjecting a metal sheet or a coated metal sheet to drawing and redrawing has been conducted from old. At the draw-redraw forming, the metal sheet shows such a plastic flow that the size of the can is increased in the height direction but the size is diminished in the circumferential direction of the can barrel.
According to the conventional redrawing method, the redraw forming is accomplished by relatively moving a punch having a diameter smaller than that of a preliminarily drawn cup drawn in advance to have a large diameter and a redrawing die so that the punch and die are engaged with each other. An annular cup-holding member is arranged within the preliminarily drawn cup, and a bottom face of the preliminarily drawn cup is held by the holding member and the flat face portion of the redrawing die. At the redrawing step, the holding member is moved synchronously with the redrawing die.
In this structure, at the relative movement of the punch and the die, the preliminarily drawn cup is draw-formed into a deep-draw-formed cup having a small diameter by a curvature corner portion of the redrawing die and simultaneously, the side wall of the preliminarily drawn cup is bent and elongated and the thickness of the side wall is reduced. At the deep-draw forming, the holding member and the flat face portion of the redrawing die act as the blank holding face to a portion of the cup on which influences of the plastic flow are imposed.
Various methods of sufficiently reducing the thickness of a deep-draw-formed cup and uniformly reducing the thickness of the side wall of the cup have been proposed in the conventional thickness-reducing redrawing process (see, for example, Japanese Unexamined Patent Publication No. 56-501442 and Japanese Unexamined Patent Publication No. 01-258822).
These thickness-reducing redrawing methods are characterized in that the curvature of the corner portion of the redrawing die is specifically adjusted, and by reducing the curvature of the corner portion, the thickness of the deep-draw-formed cup is reduced uniformly and sufficiently. However, if the curvature of the corner portion of the redrawing die is reduced, the risk of damage of the metal sheet and coating layer increases and the adhesion between the coating layer and the metal sheet tends to decrease. Accordingly, in a final canned product, troubles such as corrosion of the metal and elution of the metal are caused, and a swollen can is formed by generation of hydrogen or a leaking can is formed by pitting.
In order to obtain a large thickness reduction ratio (1--can barrel thickness/blank thickness), it is necessary to set the radius of the redrawing die and the radius R.sub.H of the holding member at smaller values or to apply a larger blank holding force. However, in the case where the surface of the material is coated, the bearing pressure imposed on the coating surface rises and it is apprehended that the coating will be fatally damaged. Furthermore, in the case where the side wall face of the preliminarily drawn cup is arranged on the blank holding face, the space from the blank holding face is equal to the maximum sheet thickness in the blank holding face, and therefore, if a cup having an uneven thickness distribution is redrawn, the blank holding force is concentrated on the peripheral side of the blank holding face where the thickness is largest, while no satisfactory blank holding effect is obtained on the inner circumferential side of the blank holding face, and traces of wrinkles are sometimes left on the redraw-formed cup.
In the case where a draw-redraw-formed cup is processed in a vessel for a canned product, the cup is then subjected to beading, necking and flanging of the side wall. In general, in the side wall of the draw-redraw-formed can barrel, work hardening is caused by plastic deformation and the yield point becomes larger than that of the starting blank, and therefore, there is observed a tendency that the above-mentioned post treatments are not advantageously performed. This work hardening is especially conspicuous in the vicinity of the open end of a cup obtained from a high-strength steel sheet as the starting blank at high draw and redraw ratios, and the above-mentioned undesirable tendency is prominent in this cup.